Monochromator and scanning electron microscope using the same

ABSTRACT

An invention providing a scanning electron microscope composed of a monochromator capable of high resolution, monochromatizing the energy and reducing chromatic aberrations without significantly lowering the electrical current strength of the primary electron beam. A scanning electron microscope is installed with a pair of sectorial magnetic and electrical fields having opposite deflection directions to focus the electron beam and then limit the energy width by means of slits, and another pair of sectorial magnetic and electrical fields of the same shape is installed at a position forming a symmetrical mirror versus the surface containing the slits. This structure acts to cancel out energy dispersion at the object point and symmetrical mirror positions, and by spatially contracting the point-converged spot beam with a converging lens system, improves the image resolution of the scanning electron microscope.

FIELD OF THE INVENTION

The present invention relates to a monochromator for making the energy of an electron beam monochrome and a scanning electron microscope having the same.

BACKGROUND OF THE INVENTION

In recent years, the scanning electron microscope in many cases utilizes a low acceleration energy within a few kilovolts to prevent the electron beam from generating an electrostatic charge on the semiconductor sample. In these types of low acceleration SEM (scanning electron microscopes), the characteristic energy width of the electron source for the acceleration energy becomes relatively large so chromatic aberrations prevent obtaining an electron beam with a sufficiently small spot.

To solve this problem, technology utilizing an energy filter to selectively allow only electrons in the desired energy range to pass was disclosed in patent documents 1 through 5.

Patent document 1 discloses technology for reducing the chromatic aberrations by installing a magnetic field type energy filter between the electron source and the objective lens. This technology combines a magnetic sector field for lateral beam input/output, with a non-dispersive point (achromatic state) used as a crossover, along with an iris (beam limiter) installed at the beam convergence point along the linear optical axis of the beam emitted from the electron source.

The technology disclosed in patent document 2 utilizes a double structure Wien filter (overlapping magnetic field and electrical field) to monochromatize the electron beam, disperse the energy of the electron beam and select energy electrons with a single slit plate (FIG. 5 and FIG. 6 of patent document 2). The technology shown in FIG. 10 of patent document 2 discloses a monochromator composed of four magnetic sector fields. In this technology, after monochromatizing the electron beam with a slit installed downstream of two prestage magnetic sector field filters, the electron beam was non-dispersively converged by two final stage magnetic sector field filters.

The technology disclosed in patent document 3 describes an energy filter (monochromator) utilizing four semispherical electric deflectors (spherical electrical fields) and having slits in the mirror symmetrical surfaces midway between those deflectors. The beam emitted from the electron gun is subjected to energy dispersion by two prestage spherical electrical sector fields. Then, the energy width is reduced by the slits, the beam is non-dispersed by passing through two final stage spherical electrical sector fields, and a crossover is formed.

The technology disclosed in patent document 4 is not a monochromator but utilizes a Ω-type magnetic field filter installed in the projector lens system of a transmittance type electron scanning microscope. This energy filter further contains a linear path for the electron beam when not analyzing energy, and is further designed not to change the excitation of the front-rear lens.

The technology disclosed in patent document 5 is also not a monochromator and instead utilizes an energy filter composed of three sectorial magnets. One magnetic field is a uniform magnetic field for lateral input/output beam emission and works in combination with other non-uniform magnetic fields for energy selection.

-   Patent document 1:

JP-A No. 195396/1999

-   Patent document 2:

JP-A No. 357809/2001

-   Patent document 3:

JP-A No. 233145/1992

-   Patent document 4:

JP-A No. 191384/1999

-   Patent document 5:

JP-A No. 37536/1995

SUMMARY OF THE INVENTION

The patent documents 1 through 5 disclosing monochromator and energy filters with an overlapping magnetic field and electrical field (patent document 2), a magnetic field (patent documents 1, 2, 4, 5) and an electrical field (patent document 3) had the following problems.

The energy filter using a magnetic field to perform filtering had the problem that when attempting to obtain secondary beam convergence by lateral irradiation onto the magnetic pole surface, secondary chromatic aberrations occurred and the scanning beam could not be sufficiently narrowed, resulting in poor resolution.

However, on monochromators that perform secondary convergence (or focusing) by a using semispherical electrical field, a semi-spherical condenser electrode must be installed along the linear optical path of the electron beam. So using a high-sensitivity scanning electron microscope without a monochromator requires that holes for these electrodes be drilled in the electrode along the optical axis of the electron beam (or the electron beam trajectory emitted from the electron source when not deflected by a deflector of an electron beam, which is hereinafter referred to as a “linear optical axis”). When used as a monochromator however, these cause distortions in the electrical field distribution that makes it impossible to sufficiently converge the electron beam.

In comparison with the technology disclosed in patent documents 1, 3, 4, and 5, the monochromator with Wien filter disclosed in patent document 2 had such a problem that when the dispersion is to be enlarged, the optical axis of the electron beam becomes long, and resolution deteriorates due to a drop in vibration resistance.

An object of the present invention is therefore to provide a monochromator and a scanning electron microscope using that monochromator capable of high resolution, by suppressing higher-order aberrations, without causing a drop in beam convergence.

To achieve the above objects, the present invention proposes a monochromator, and a scanning microscope utilizing that monochromator for deflecting the electron beam from the optical beam linear optical axis by using sectorial magnetic fields, and dispersing the energy of the beam and allowing electron passage through a slot. The specific structure, application and effect of the present invention are explained next in the detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing for describing the scanning optical microscope comprising the monochromator of the embodiment of the invention;

FIG. 2 is a drawing for describing the track in the x direction of the electron beam within the monochromator;

FIG. 3 is a drawing for describing the track in the y direction of the electron beam within the monochromator;

FIG. 4 is a drawing for describing the energy dispersion trajectory of the electron beam within the monochromator;

FIG. 5 is a drawing for describing the electro-optical parameters;

FIG. 6 is a graph showing the primary aberration coefficient of the monochromator;

FIG. 7 is a graph showing the secondary aberration coefficient of the monochromator;

FIG. 8 is a drawing showing the focus point adjustment method for the monochromator;

FIG. 9 is a drawing for showing the beam shape on the mass separator slit;

FIG. 10 is a drawing showing the fan-type toroidal electrode shape on the present monochromator;

FIG. 11 is a graph showing the track of the monochromator for describing the second embodiment of the present invention;

FIG. 12 is a graph showing the primary aberration coefficient of the monochromator for describing the second embodiment; and

FIG. 13 is a graph showing the secondary aberration coefficient of the monochromator for describing the second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The embodiment of the present invention described next is capable of deflecting a magnetic field to make an electron beam travel along an optical axis, and deflecting an electron beam away from an optical axis; and combined with electrical field deflection is capable of focusing a beam perpendicular to the electron beam deflection direction without generating higher-order aberrations.

First Embodiment

The optical system and optical system operation for the scanning electron microscope containing a monochromator of the first embodiment of the present invention is described next while referring to FIG. 1.

When a voltage is applied across an electron source 1 and anode electrode 2, an electron beam 3 is emitted from the electron source 1 along the linear optical axis. This electron beam 3 also includes an electron beam 3 a widening to several dozen millirads, coupling the actual image of the electron source to an internal section of the monochromator 20 with a first convergence lens 4. The electron beam is then deflected in a range of 140 degrees or more and 170 degrees or less, by a magnetic field generator 21 of the prestage deflector system, and by deflecting to the same angle in the opposite direction by a first electrical field generator 22, the beam is placed in a direction parallel to the original linear optical axis, focused at the slits 24 position in the x direction (left and right on paper surface) and energy selection performed.

A non-dispersed ghost image is then formed to cancel out the energy dispersion generated in the prestage deflection system. This ghost image is formed by a second electrical field generator 23 in the final stage deflection system installed at a position symmetrical to the surface containing slits for deflecting the electron beam to return to the linear track of the electron beam, and by point convergence by the magnetic field generator 21 along a mirror symmetrical trajectory. This monochromatized ghost image of the electron source is formed into an image by a second conversion lens 6 and then reduced by an objective lens 8, to form a tiny crossover on the surface of a sample 9.

The angle opening of the primary electron beam 3 or amount of beam current is restricted by a beam limiter 5 installed downstream of the monochromator 20. This electron beam 3 is further two-dimensionally scanned on the sample 9 by the scanning coil 7 (deflector). The magnetic field generator and electrical field generator comprising the monochromator 20 are preferably formed so that the electrons form a fan shape widening along the electron beam path.

The electrons 10 (secondary electrons, and/or reflected electrons) emitted from the sample, rise while receiving the effect from the objective lens 8. These electrons 10 rise and strike the conversion electrode 11 and generate new secondary electrons 12. These secondary electrons 12 strike the scintillator 13 applied with a positive high voltage and emit light. After these electrons are converted into electrical signals and amplified by a photomultiplier tube, they can be observed as a microscope image (SEM image) on the cathode ray tube. The embodiment described in FIG. 1, employed a technique that converted electrons emitted from the sample into secondary electrons with a conversion electrode and then detected these electrons with a detector. However, the invention is not limited to this example and electrons emitted from the sample may for example be supplied directly to a detector.

The optical devices described with FIG. 1 are connected to a control device not shown in the drawing. This control device adjusts the voltage and the electrical current applied to each optical element (or device).

The track of the electron beam of the electro-optical system for the monochromator in the following embodiment is next described in detail while referring to FIG. 2 through FIG. 4.

FIG. 2 shows the electron beam 3 a widening in the radial (x direction) after deflection by electron beam 3 in the monochromator 20. The monochromator 20 is composed of flat surfaces that have mirror symmetrical upper and lower flat surfaces (or two-fold symmetry) containing the slits 24.

The drive (excitation) of the first focus lens 4 is adjusted and the electron beam 3 beam is input to couple the actual image of the electron source at the crosspoint with the mirror symmetrical surface 18 and electron beam 3. The beam is initially deflected approximately 160 degrees by driving the magnetic field generator 21. The actual image is present along this deflection trajectory. The beam next passes through the drift space and is next deflected one time at an identical angle in the opposite direction by the first electric field generator 22, and is converged in the x direction onto the slit 24 position. The beam is then made to travel along a symmetrical track by the magnetic field generator 21 and second electric field generator 23 of the final stage deflector system installed on mirror symmetrical positions versus the surface containing the slits 24. The beam then disperses outward after being emitted from the magnetic field generator 21. A ghost image however is formed on the mirror symmetrical surface 18 by the emitted electron beam 3 as seen from the beam limiter 5.

The magnetic field generator 21 of the present embodiment is formed from two magnetic poles arrayed at perpendicular intervals on the paper. By forming magnetic fields along a direction from the closer edge to the farther edge of the paper (away from the closest side) between these two magnetic poles, (in a direction perpendicular to optical axis of electron beam), the electron beam is deflected towards the left side of the paper surface.

The input/output beam surface of the sectorial magnetic generators is set largely perpendicular to the electron beam. There is no focusing effect (y direction) on the direction the magnetic lines of force are facing.

The example given in this embodiment described the magnetic pole deflecting the electron beam outside the linear optical axis, integrated with another magnetic pole for returning the beam to the linear optical axis after it has passed the second electrical field generator. However, the present invention is not limited to this example and deflection may be performed using respectively different magnetic poles.

The device of the present embodiment can be utilized as a normal scanning electron microscope that does not monochromatize the electron beam and therefore comprises a means to turn the drive (excitation) to the magnetic field generator 21 on and off, as well as a means to erase the residual magnetic fields.

FIG. 3 shows the widening electron beam in the perpendicular direction (y direction) of the paper of FIG. 2 converted to a straight line extending along a curved trajectory in the z direction. Because the electron beam 3 is perpendicularly beamed into and out of all the sectorial magnetic fields, there is no convergence effect when being beamed inwards or outwards. However, by forming the electric field generator as a toroidal electrical sectorial field, a converging effect is generated in the y direction. First of all, the widened electron beam 3 a is focused by the first converging lens 4, and converged in the y direction along the deflection track of the fan shaped magnetic field sectors formed by the magnetic field generator 21. The beam transits the drift space, and passes the electrical field formed by the first electrical field generator 22, simultaneously point-converging in the x direction at the slit 24 position. The electron beam 3 that passed through the slits 24 is formed in a mirror symmetric electron trajectory by the electrical field formed by the second electrical field generator 23, and by the magnetic field formed by the magnetic field generator 21 that were installed mirror symmetric to each other.

In the present embodiment, using a toroidal electrical field formed by a sectorial toroidal electrode for converging in the y-direction is effective in suppressing the generation of higher-order aberrations, compared to the case where lateral beam input/outputs of the sectorial magnetic field are converged in the y direction.

In other words, converging the beam in the y direction by using an ordinary sectorial magnetic field, utilizes the effect generated from laterally irradiating an electron beam onto or from the plane of a magnetic field with sudden changes in the input beam surface (incidence plane) or output beam surface. Converging the beam in this way therefore creates secondary aberrations involving the widening beam angle. In the present embodiment, however, the y axis convergence effect is attained only by using a toroidal electrical field so that the electron beam can be perpendicularly irradiated onto or from a magnetic deflector device and the generation of secondary aberrations can therefore be suppressed.

The magnetic field generator of the present embodiment is composed of a first input surface for inputting an electron beam from the linear optical axis, a first output surface for emitting the electron beam input from the first input surface, a second input surface for inputting the electron beam deflected by the second electric field generator 23, and a second output surface for emitting the electron beam input from the second input surface, along the linear optical axis.

Also in this embodiment, the electron beam is deflected from the linear optical path by using sectorial magnetic fields. Compared to deflection using a sectorial electrical field, the dimensions along the linear guide can be shortened when deflecting with a sectorial magnetic field. In other words, in order for sectorial electrodes to be installed along the electron beam path, the electron beam path is required to be separated into energy filter prestages and latter stages. Sectorial magnetic fields are, however, not subject to this limitation and can be brought into proximity with the shuttling electron beam or crossed as shown in FIG. 1, and the height of the device can be lowered, then the shaking resistance can be improved and better image resolution achieved.

FIG. 4 shows the energy dispersion trajectory 3 d. The energy dispersion trajectory 3 d is emitted at the point of beam input from the sectorial magnetic field by the magnetic field generator 21, passes through the first sectorial toroidal electrical field generated by the first electrical field generator 22 and reaches a maximum in the vicinity of the slit 24.

The energy dispersion trajectory 3 d beam then using a mirror symmetrical trajectory, and energy dispersion diminishes and becomes zero in the ghost image of the electron beam 3 d emitted from magnetic field generator 21 on the mirror symmetrical surface 18 as seen from the beam restrictor 5. In summing up the above, point convergence and triple convergence of the non-dispersed energy (dispersion is zero) are achieved in the ghost image.

A trajectory calculation program called, TRIO (T. Matsuo, H. Matsuda et al.; Computer Program “TRIO” for Third Order Calculation of Ion Trajectory, Mass Spectrometry 24 (1976), pp. 19–62) was used to calculate the convergence characteristics of this monochromator. The parameters are defined in FIG. 5. The distances in the drift space are L1, L2, L3, the deflection path radius in the electrical field and magnetic fields are R1, R2 and the deflection angles are θ1, θ2. In the monochromator of the present invention, the mirror symmetrical surface is set as a two-fold (upper/lower) symmetry. If a monochromator is designed with each parameter having the same ratios as above then its performance will be the same as this embodiment.

When the size (object point) of the actual image as seen from the beam input side, or in other words, when the size of the electron beam (x₁, y₁) is small enough to be ignored then, when the widening beam angles are α and β, and the energy width was set as δ(=ΔE/E), the beam width x₂ in the x direction, and the beam width y₂ in the y direction after passing through the monochromator lens system are expressed by the following quadratic formulas. x ₂ =A*α+D*δ+AA*α ² +BB*β ₂  (1) y ₂ =B*β+AB*αβ  (2)

Here, A is the x-direction aberration coefficient, B is the y-direction aberration coefficient, D is the energy dispersion coefficient, AA, BB, AB are respectively secondary aberration coefficients relating to the open angle of the beam spaces. When the above parameters with the values in the boxes at the lower part of FIG. 5 are established as the specific embodiment of the present invention, the changes in the primary aberration coefficients and secondary aberration coefficients are shown in FIG. 6 and FIG. 7. The horizontal axis indicates the progression direction of the z axis. The left end (0) in the figure is the theoretical object point for the beam input side, and the right end (10) is the ghost image point on the beam output side. In other words, the right end (10) is the monochrome convergence point.

As clearly shown in FIG. 6, with the aberration coefficient A for the x-direction open angle α, and the aberration coefficient B for the y-direction open angle β simultaneously converging (A=B=0), the energy dispersion coefficient D reaches −0.04. The x-direction and y-direction simultaneously converge on the monochrome convergence point at the right end (10), moreover the dispersion is canceled out. In this way, in the primary formula for the trajectory calculation, the monochromator of the present invention is triple-converged (A=B=D=0) both in terms of space and energy.

In the secondary aberration coefficient of FIG. 7, the three secondary aberration coefficients have been eliminated at the right end monochrome convergence point (AA=BB=AB=0). This is the effect of mirror symmetry at the monochromator slit position, and perpendicular input of the beam to the plane of the sectorial magnetic field.

As described above, if the monochromator of the present invention is installed below the first focus lens of the scanning electron microscope, all of the monochrome convergence points converged using the primary and secondary aberration coefficients A, B, D, AA, BB, AB can be obtained after limiting the beam energy at the slits. These are set as object point for the second focus lens, and by contracting (the focus point) with the objective lens, an extremely small spot with few chromatic aberrations can be obtained and the resolution of the scanning electron microscope improved.

The method for adjusting the monochromator of the present invention is described next while referring to FIG. 8.

(A) First of all, the drive (excitation) of magnetic field generator 21 on the monochromator 20 is cut off and an actual image of the electron source is formed on the mirror symmetric surface 18. A beam restrictor 14 with a diameter of approximately 500 micrometers is installed for that purpose. By then using the adjustment scanning coil 4 a to perform two-dimensional scanning with electron beam 3 s with this beam restrictor 14, a condensed SEM image can be acquired, and an actual image obtained under optimal image resolution conditions. Adjusting the second focus lens and objective lens with these conditions allows optimizing the SEM image resolution of the sample.

(B) Next, as the drive (excitation) of the magnetic field generator 21 is successively amplified, an electron beam 3 t is irradiated onto the electrode of a first electrical field generator 22 only when the drive or excitation is within a certain range. By setting the excitation current of magnetic field generator 21 to an intermediate value for the current flowing into the external electrode, it is possible to make adjustments based on accurate, optimal excitation conditions.

(C) The electron beam 3 u is irradiated onto the electrode of a second electrical field generator 23 when the voltage of the first electrical field generator 22 is increased. By measuring the electrical current flow into the external electrode, it is possible to find the conditions needed for applying a voltage for irradiating the center of the gap of the second electrical field generator 23 with an electron beam. This enables to make adjustments based on accurate voltage application conditions the same as when adjusting the sectorial magnetic field.

(D) Set the second electrical field generator 23 to the same voltage conditions as the first electrical field generator 22.

(E) Compare with the SEM image of the sample in (A), and make fine adjustments to the voltage of the first electrical field generator 22 and second electrical field generator 23 to minimize defocusing or image shifting.

(F) Insert a beam restrictor 24 up to the crossover 19 of the electron beam. Make fine adjustments using the scanning coil 4 a so that beam restrictor 24 is in the center of the image.

The reference numerals 3, 3 d and 3 e in FIG. 9 represent results calculated for beam shape using TRIO under the conditions for focusing the electron beam at the beam restrictor 24 position. A beam shape with a wide energy width is a dispersion image such as represented by 3 f. Although the constriction in beam width in the y direction is small relative to a hole of approximately 4 micrometers, a widening effect occurs in the x-direction due to the diffusion of energy, making it impossible to observe clear and sharp focused image. There is, however, little change in energy dispersion due to focus adjustment so fine adjustments may be made to the sectorial toroidal electrical field under the condition that the lateral width of the entire image is minimum. Finally, check that the monochrome point on the mirror symmetric surface 18 matches the actual image formed prior to excitation by the sectorial magnetic field of the magnetic field generator 21 as seen from the output slit of monochromator 20. This method makes the optical conditions downstream of the monochromator completely identical, and fine adjustments made to the second electrical field generator 23 parameters prevent image shifts and defocusing from occurring before or after excitation of the magnetic field generator 21.

A specific method for adjusting the parameters of the sectorial toroidal electrical field is described while referring to FIG. 10. By making the electrical field into a toroidal electrical field, a convergence effect is generated in the y-direction. Two adjustment terminals 22 c and 22 d are installed in the y-direction for making fine adjustments to the toroidal electrical field. Applying the same voltage Vc to these electrodes makes tiny changes in the toroidal constant, for changing the y-direction convergence point. The focus point can also be adjusted in the y-direction, not only in the x-direction.

The electrodes 22 a, 22 b that comprise the sectorial toroidal electrical field of the present embodiment have a curvature along the electron beam trajectory. These electrodes 22 a, 22 b further have a shape with a particular curvature radius Ry along the y-direction of their cross section. Electrodes shaped in this way can generate a convergence effect in the y-direction.

These electrodes 22 a, 22 b and the adjusting electrodes 22 c, 22 d are respectively connected to a variable power supply (not shown in drawing).

Second Embodiment

The second embodiment of the invention is described next. The energy dispersion trajectory 3 d is shown in FIG. 11. The energy dispersion trajectory is emitted at the point in time that the first sectorial magnetic field formed by the magnetic field generator 21 is irradiated. This becomes uniform after passing the first sectorial electrical field formed by the first electrical field generator 22 and passes through the beam restrictor 24. After then passing along the mirror-symmetrical trajectory, the energy dispersion diminishes and becomes completely zero at the point of emission from the sectorial magnetic field formed by the magnetic field generator 21. Even after emission, the energy dispersion of the electron beam 3 d remains unchanged at zero. Summing the above, since the energy dispersion along the linear optical axis is always zero, the beam is subjected to point convergence and triple convergence from energy non-dispersion regardless of the point convergence position. The optical parameters for this kind of completely monochrome conditions are summarized in the lower half of FIG. 11. In a monochromator configured with these values, even if the convergent point position has shifted from a change in the parameters due to other optical elements on the optical axis, stable monochrome conditions can always be restored and, thus, it is easy to adjust the device.

The changes in the aberration coefficients when the above parameters were set are shown in FIG. 12 and FIG. 13. As FIG. 12 clearly shows, while the aberration coefficient A for the x direction open angle α of and the aberration coefficient B for the y direction of open angle β are simultaneously converged (A=B=0) on the beam restrictor 24 position (5), the energy dispersion coefficient D is reaching −0.04 the same as in the previous embodiment. Simultaneous convergence is taking place in the x direction and y direction at the monochrome convergence point on the right end (10). Furthermore, the dispersion is canceled out immediately after emission from the fan shaped magnetic field. The monochromator of the present invention is in this way triple-converged (A=B=D=0) in the primary formula for the trajectory calculation, both in terms of space and energy. It should be noted in particular that D is always at zero from the emission point (9) of the sectorial magnetic field onwards. Further, in the secondary aberration coefficient changes as shown in FIG. 13, the three aberration coefficients are all eliminated (AA=BB=0) at the monochrome point on the right end (10). This is the effect of mirror symmetry at the monochromator slit position (5), and the perpendicular input of the beam perpendicular to the plane of the sectorial magnetic field.

The embodiments of the present invention allow high resolution in a low acceleration SEM where it was previously impossible to obtain a resolution within 2 nanometers. Though Schottky emission electron sources and thermal field emission electron sources offered superior characteristics in terms of continuous stable operation over several thousand hours, these devices had the problem that energy width was relatively large at approximately 0.6 eV and also large problems with chromatic aberrations compared to cooled-cathode discharge electron sources. However, both long term stable operation and reduced chromatic aberrations can be achieved by utilizing the monochromator described in the present embodiments.

The example in the embodiments of the present invention described a scanning electron microscope (SEM) using a monochromator at a low acceleration voltage. However, the present invention may also be applied to scanning transmission type electron microscopes (STEM) and high acceleration scanning electron-microscopes.

As described above, by installing the monochromator of the embodiment of the present invention in a scanning electron microscope, a monochromatized convergence or focus point can be obtained for all primary and secondary aberration coefficients converged after limiting the beam energy with slits. By setting this point as the object point for the focus lens and reducing it with the objective lens, an extremely tiny spot with few chromatic aberrations can be obtained and the resolution of the SEM improved.

There is also absolutely no need to change optical conditions other than for the monochromator either before or after monochromator operation so that high reliability along with simple maintenance is achieved making the invention particularly effective for use on semiconductor device production lines.

The invention as described above, renders the effect of achieving high image resolution while suppressing the generation of higher-order aberrations without lowering the beam focusing effect. 

1. A monochromator comprising: a magnetic field generator to deflect an electron beam; a first electrical field generator to further deflect said electron beam deflected by said magnetic field generator; a slit for filtering the energy of said electron beam deflected by said first electrical field generator; and a second electrical field generator for deflecting to return said electron beam that was energy-filtered by said slit.
 2. A scanning electron microscope containing an objective lens for focusing an electron beam emitted from an electron source onto a sample, wherein a monochromator is installed in said microscope for deflecting said electron beam using a magnetic field generator installed between said electron source and said objective lens, and deflecting said deflected electron beam utilizing a first electrical field generator to disperse the energy and allow the beam to pass through said slit.
 3. A scanning electron microscope according to claim 2, said monochromator comprising a prestage deflector system composed of a magnetic field deflector and electrical field deflector, and a latter-stage deflector system composed of said electrical field deflector and said magnetic field deflector installed symmetrically with respect to said prestage deflector system, and a slit formed on a mirror symmetrical surface midway between said prestage deflector system and said latter-stage deflector system, wherein along with spatially converging the beam at the slit position with said prestage deflector system in the dispersion direction (x), after limiting the energy width with the slit, the beam is once again spatially point-converged by said latter-stage deflector system and the energy dispersion canceled out.
 4. A scanning electron microscope according to claim 3, said prestage deflector system containing a magnetic field generator to deflect the electron beam outside the linear optical axis of said electron beam, and an electric field generator for deflecting said electron beam deflected by said magnetic field generator towards said slit; and said latter-stage deflector system containing an electrical field generator to deflect said electron beam that passed said slit, towards said linear optical axis, and a magnetic field generator to deflect the electron beam deflected by said electrical field generator along said linear optical axis.
 5. A scanning electron microscope according to claim 3, wherein the electrode and magnetic pole form said electrical field generator and magnetic field generator are respectively formed in a fan shape.
 6. A scanning electron microscope according to claim 5, wherein said sectorial magnetic field generator and said sectorial electrical field generator have completely equal deflection angles for said electron beam.
 7. A scanning electron microscope according to claim 5, wherein the magnetic pole forming said magnetic field generator contains a perpendicular beam input surface and beam output surface for said electron beam.
 8. A scanning electron microscope according to claim 2, wherein the theoretical object point for the electron beam input into said monochromator spatially matches the ghost image point emitted by said monochromator.
 9. A scanning electron microscope according to claim 8 containing a first focus lens and second focus lens to focus the electron beam, wherein said monochromator is installed between said first focus lens and said second focus lens, and said first focus lens forms the theoretical object point for the electron beam input to said monochromator, and the ghost image point of the electron beam emitted from said monochromator is the object point of said second focus lens.
 10. A scanning electron microscope according to claim 9, wherein the excitation conditions for said first focus lens and said second focus lens switching do not change, even before and after switching between excitation and with no excitation of the sectorial magnetic field in the monochromator.
 11. A scanning electron microscope according to claim 3, wherein said monochromator contains a prestage deflector system, and a latter-stage deflector system installed symmetrically with said prestage deflector system, and a beam restrictor installed between their mirror symmetrical surfaces, and along with spatially converging the dispersion direction (x) , and simultaneously converging in that perpendicular y direction at the beam restrictor position in said prestage deflector system; after the energy width is simultaneously limited by the beam restrictor, said latter stage again spatially point converges the beam and cancels out the energy dispersion.
 12. A scanning electron microscope according to claim 5, wherein said sectorial electrical field formed by said electrical field generator is a toroidal electrical field with a curvature in the perpendicular y direction relative to the deflection direction.
 13. A scanning electron microscope according to claim 12, containing a monochromator wherein the shapes of the magnetic field and toroidal electrical field of said prestage deflector system, and toroidal electrical field and magnetic field of the latter stage deflection system of the monochromator are mirror symmetric to the intermediate surface; and the aberration coefficients A, B relating to the energy dispersion coefficient D, an open angle α of beam direction x, an open angle β of beam direction y, are converged to zero, and also the secondary aberration coefficients AA, BB, AB relating to the open angle α and open angle β are converged to zero at the ghost image point of the beam emitted from said monochromator.
 14. A scanning electron microscope having an electron source, and an objective lens for focusing an electron beam emitted from said electron source onto a sample, wherein said scanning electron microscope further have a monochromator comprising: a magnetic field generator for generating a magnetic field perpendicular to the irradiation direction of said electron beam and having at least two magnetic poles between said focusing lens and objective lens for deflecting said electron beam; a first electrical field generator for deflecting said electron beam deflected by said magnetic field generator, in a direction parallel with the irradiation direction of said electron beam; a slit for filtering the energy of said electron beam deflected by said first electrical field generator; and a second electrical field generator for deflecting the electron beam that passed through said slit, and wherein the electron beam deflected by said second magnetic field generator is deflected so as to return to the original electron beam trajectory.
 15. A scanning electron microscope having an electron source, and an objective lens for focusing an electron beam emitted from said electron source onto a sample, wherein said scanning electron microscope further has a monochromator, installed between said focusing lens and objective lens, comprising: a magnetic pole for generating a magnetic field to deflect said electron beam; a first electrode for generating an electrical field for deflecting said deflected electron beam towards a slit for filtering energy; and a second electrode for deflecting the electron beam that passed through said slit towards said pole, and wherein said monochromator starts energy dispersion of said electron beam at the electron beam input surface of said magnetic pole, awaits a fixed level of dispersion after emission from said first electrode, and after energy selection at the slit, the dispersion decreases after beam input to said second electrode, and is set so that energy dispersion from the electron beam emission surface of said magnetic pole is maintained at zero.
 16. A scanning electron microscope having an electron source, and an objective lens for focusing an electron beam emitted from said electron source onto a sample, wherein said scanning electron microscope further has a monochromator comprising: a magnetic field generator for generating a magnetic field to deflect said electron beam; a first electrical field generator for generating an electrical field to deflect the electron beam emitted from said magnetic field generator toward a slit for selecting the energy of said electron beam; and a second electrical field generator for deflecting the electron beam that passed through said slit to return to the original electron beam trajectory, wherein said magnetic field generator has: a first beam input surface for irradiation by said electron beam; a first beam output surface for emitting said electron beam deflected by said magnetic field generator; and a second beam input surface for inputting the electron beam deflected by said second electrical field generator; and a second beam output surface for emitting the electron beam input from said second beam input surface, wherein said beam input surfaces and beam output surfaces of said electron beam are formed to be perpendicular to the trajectory of said electron beam.
 17. A scanning electron microscope having an electron source, and an objective lens for focusing an electron beam emitted from said electron source onto a sample, comprising: a magnetic field generator to generate a magnetic field for deflecting said electron beam outside the linear optical axis; a first electrical field generator to generate an electrical field for deflecting said electron beam emitted from said magnetic field generator towards said slit for filtering the energy of said electron beam; and a second electrical field generator for deflecting the electron beam that passed through said slit, towards said optical axis; wherein said first electrical field generator has a toroidal electrode for converging said electron beam in the x-direction, and also for converging said beam in the y-direction perpendicular to said x-direction, and a pair of adjusting electrodes for varying the electrical field in said y direction.
 18. A scanning electron microscope according to claim 17, wherein said second electrical field generator has a pair of adjusting electrodes for making tiny changes in the electrical field in the y-direction to become symmetrical with said first electrical field generator when said slit surface is a symmetrical surface. 